The present invention relates to novel aromatic copolyesters capable of being subjected to melt-processing having high strength and high modulus and less anisotropy of mechanical properties.
Recently there has been increasing demand for materials which when formed into fibers, films and shaped articles exhibit high modulus and resistance to heat and to chemicals. Although polyesters are in use for various shaped articles, many of them are inferior in mechanical properties such as flexural modulus and have therefore been unsuitable heretofore for uses requiring high strength and high modulus. As a method for improving such mechanical properties there has been known incorporating fillers or reinforcing materials such as calcium carbonate or glass fibers into polyesters. However, this method causes problems in practical use; for example, the light weight characteristic, which is a feature of plastics, is lost because of increase in specific gravity of the resulting blend, and when the blend is processed, the processing machine is subjected to heavy wear.
Liquid crystal polyesters have come to be noted recently as polyesters suitable for uses requiring high strength and high modulus even without using reinforcing materials or the like. Special attention has been paid to such polyesters since W. J. Jackson made public thermoplastic liquid crystal polyesters comprising polyethylene terephthalate and p-hydroxybenzoic acid in U.S. Pat. No.3,804,805 and Journal of Polymer Science Polymer Chemistry Edition, Vol.14, page 2043 (1976). Since then, various liquid crystal polyesters have been developed and studied with a view to attaining both improvement of strength and modulus, and melt-processability. However, none of them have been put to practical use yet as shaped articles or films. This is because these liquid crystal polymers exhibit a high orientatability in molten condition resulting in creation of a great anisotropy in their mechanical properties.
As means for eliminating such anisotropy and attaining a high modulus and high strength biaxial oriented film, the use of a cholesteric liquid crystal polymer has been proposed by W. R. Krigbaum et al (see U.S. Pat. No.4,412,059). According to this proposed method, however, the transesterification is slow and it is impossible to introduce a desired amount of optical active units. Further, in some particular linkage of flexible structural units there may arise problems such as deterioration of physical properties or spoiling the formation of the liquid crystal.